Fabrication of Flux Qubit with a Mechanical Degree of Freedom

We report on the fabrication and characterization of superconducting flux qubit coupled to nanomechanical resonator vibration modes. Anisotropic and isotropic etching processes parameters of plasma created by a reactive ion etching of a CF₄ gas, were optimized to suspend one arm of the qubit. One of the beams was characterized using a magnetomotive detection scheme in the transmission regime. And suspended beams with different length coupled to a superconducting flux qubits were characterized at base temperature by performing spectroscopy measurements.     

Dispersive Charge and Flux Qubit Readout as a Quantum Measurement Process

We analyze the dispersive readout of superconducting charge and flux qubits as a quantum measurement process. The measurement oscillator frequency is considered much lower than the qubit frequency. This regime is interesting because large detuning allows for strong coupling between the measurement oscillator and the signal transmission line, thus allowing for fast readout. Due to the large detuning we may not use the rotating wave approximation in the oscillator-qubit coupling. Instead we start from an approximation where the qubit follows the oscillator adiabatically, and show that non-adiabatic corrections are small. We find analytic expressions for the measurement time, as well as for the back-action, both while measuring and in the off-state. The quantum efficiency is found to be unity within our approximation, both for charge and flux qubit readout.      

Angular Momentum Balance on Light Reflection

Abstract

It was pointed out by Holbourn in 1936 that there is an apparent breakdown of angular momentum conservation on reflection of circularly polarized light at the plane interface between two lossless dielectrics. The analysis presented in this paper shows that this discrepancy does not arise provided that: (a) the intensities of the incident, reflected, and transmitted beams vanish at large distances from their axes, and (b) the angular momentum of each beam is calculated from first principles.     

Dynamics of a Flux Qubit Coupled with a Quantized Electromagnetic Mode of a Lossy Cavity

The dynamics of a rf-SQUID qubit placed within a 1-D lossy resonant cavity is reported. Initially preparing the combined system in a state with no more than one excitation, the effects of loss mechanisms can be carefully analyzed, for different values of the cavity quality factor Q. The results here discussed demonstrate the possibility of controlling these phenomena in such a way to be able to observe “coherent” Rabi oscillations and the periodically generation of factorized and entangled states (with a priori known characteristics). The importance of these results in the general context of quantum communications is carefully discussed as well as some technical detail necessary for the experimental feasibility of my theoretical scheme.     

Orientation of the Angular Momentum in Superfluid 3He-A in a Stretched Aerogel

Superfluid ³He-A in a fully characterized stretched aerogel, used in previous work by Pollanen et al., has been studied for parallel and perpendicular orientations of the magnetic field relative to the anisotropy axis of the aerogel. Consistently, we find that an equal spin pairing state (ESP) is stabilized down to the lowest temperature. From our pulsed NMR frequency shifts as a function of temperature and tip angle, the orientation of the orbital angular momentum $\hat{l}$ has been determined. The aerogel anisotropy introduced by uniaxial stretching tends to align $\hat{l}$ in the axial state parallel to the strain axis, consistent with the theory proposed by Sauls and contrary to Volovik’s prediction based on an impurity calculation of Rainer and Vuorio.     

Multistate Switching of Photonic Angular Momentum Coupling in Phase-Change Metadevices

The coupling between photonic spin and orbital angular momenta is significantly enhanced at the subwavelength scale and has found a plethora of applications in nanophotonics. However, it is still a great challenge to make such kind of coupling tunable with multiple sates. Here, a versatile metasurface platform based on polyatomic phase-change resonators is provided to realize multiple-state switching of photonic angular momentum coupling. As a proof of concept, three coupling modes, namely, symmetric coupling, asymmetric coupling, and no coupling, are experimentally demonstrated at three different crystallization levels of structured Ge₂Sb₂Te₅ alloy. In practical applications, coded information can be encrypted in asymmetric mode using the spin degree of freedom, while revealing misleading one without proper phase change or after excessive crystallinity. With these findings, this study may open an exciting direction for subwavelength electromagnetics with unprecedented compactness, allowing to envision applications in active nanophotonics and information security engineering.     

Experiments on a Superconducting Qubit Manipulated by Fast Flux Pulses: The Issue of Non-adiabaticity

The double SQUID qubit is a superconducting interferometer (SQUID) made of two Josephson junctions and two superconducting loops. Its energy potential can be greatly modified in shape and symmetry by using two magnetic control fluxes that can change the potential from a double well to an almost harmonic single well: This feature is exploited for manipulating the qubit without resorting to the usual NMR-like techniques with microwave irradiation. The qubit machinery relies on these operations being performed non-adiabatically, realizing a transition between the two lowest-lying energy levels, at the same time avoiding excitation of upper levels, a condition that can be satisfied by using control pulses with proper risetime. We show experimental results referring to manipulation of the qubit at different rates and make a qualitative comparison with theoretical expectations.     

Commutation Rules and Eigenvalues of Spin and Orbital Angular Momentum of Radiation Fields

Abstract

We investigate the separation of the total angular momentum J of the electromagnetic field into a ‘spin’ part S and an ‘orbital’ part L. We show that both ‘spin’ and ‘orbital’ angular momentum are observables. However, the transversality of the radiation field affects the commutation relations for the associated quantum operators. This implies that neither S nor L are angular momentum operators. Moreover their eigenvalues are not discrete. We construct field modes such that each mode excitation (photon) is in a simultaneous eigenstate of S _z and L _z. We consider the interaction of such a photon with an atom and the resulting effect on the internal and external part of the atomic angular momentum.     

Measurement Device Independent Quantum Key Distribution Based on Orbital Angular Momentum under Parametric Light Source

On the one hand, existing measurement device independent quantum key distribution (MDI-QKD) protocols have usually adopted single photon source (SPS) and weak coherent photon (WCP), however, these protocols have suffered from multi-photon problem brought from photon splitter number attacks. On the other hand, the orbital angular momentum (OAM)-MDI-QKD protocol does not need to compare and adjust the reference frame, solving the dependency of the base in the MDI-QKD protocol. Given that, we propose the OAM-MDI-QKD protocol based on the parametric light sources which mainly include single-photon-added-coherent (SPACS) and heralded single-photon sources (HSPS). Due to the stability of OAM and the participation of parametric light sources, the performance of MDI-QKD protocol gradually approaches the ideal situation. Numerical simulation shows that compared with WCP scheme, HSPS and SPACS schemes have increased the maximum secure transmission distance by 30 km and 40 km respectively.     

Measurements of Decoherence Times in a Josephson Charge Qubit Using Fast Pulses

We study decoherence of a Josephson charge qubit using fast pulses to perform qubit rotations. The gate charge dependence of the decoherence rate indicates that the low frequency fluctuations affecting the qubit are from charges distributed with a 1/f-type spectrum. Assuming the form S_q(ω) = α/|ω|, we find $\sqrt \alpha$ = 4 × 10^?3e, which is slightly higher than the value found from very low frequency noise measurements. PACS numbers: 03.67.-a, 75.40.+r, 85.25.Cp.     

The Effect of Magnetic Field on a Quantum Rod Qubit

The Hamiltonian of a quantum rod (QR) with an ellipsoidal boundary is given after a coordinate transformation, which changes the ellipsoidal boundary into a spherical one. We then study the eigenenergies and eigenfunctions of the ground and the first excited states of an electron strongly coupled to the LO-phonon in the QR under a magnetic field. The present system may be used as a two-level qubit. When the electron is in the superposition state of the ground and the first excited states, we obtained the time evolution of the electron probability density oscillating in the QR. It is found that the magnitude of the probability density is increased by the magnetic field, whereas it decreases the oscillation period of the probability density. The oscillation period is a increasing function of the ellipsoid aspect ratio, the transverse and longitudinal effective confinement lengths of the QR, but a decreasing one of the electron-phonon coupling strength and the cyclotron frequency of the magnetic field.     

Magnetic and Electric Control of Circularly Polarized Emission through Tuning Chirality‐Generated Orbital Angular Momentum in Organic Helical Polymeric Nanofibers

Circularly polarized light emission promotes the development of smart photonic materials for advanced applications in chiral sensing and information storage. The orbital angular momentum is a unique property for organic chiral helical materials. In this work, a type of organic chiral polymeric nanowires is designed with strong chirality induced orbital angular momentum. Under the stimulus of an external magnetic field of 600 mT, circularly polarized emission from the chiral polymeric nanowire becomes more pronounced, where the g factor increases from 0.21 to 0.3. The observed phenomena mainly originate from the chirality‐dependent orbital angular momentum. Moreover, the orbital angular momentum in helical chiral nanowire structures can be suppressed by inhibiting electron transport in a helical way to diminish circularly polarized light emission at room temperature.     

Aspects of Qubit Dynamics in the Presence of Leakage

Nano-electronics devices such as small-capacitance Josephsonjunction circuits appear to be promising systems to implementquantum logic. While the elementary unit of a quantum computeris a two-state system (the qubit) the computational space of aJosephson junction qubit is a subset of a larger Hilbertspace. Therefore quantum leakage may occur, i.e. during theoperation of the device the quantum state may escape partiallyfrom the computational subspace. We study the consequences ofleakage for the fidelity of Josephson qubits and discuss howthe gate design can be optimized.     

Momentum measurements of electrons in a heavy-liquid bubble chamber

The momentum reconstruction of high-energy electrons in a heavy-liquid bubble chamber is studied using Monte Carlo simulations. The energies of the simulated electrons used in this study ranged from 10 to 100 GeV. The angular distribution and multiplicity of converted bremsstrahlung gammas is given as a function of energy for these events. An energy-independent method of correcting for poor measurements due to hard bremsstrahlung is found. This method consists of summing the reconstructed electron momentum and the momentum of the highest energy converted gamma.     

Momentum in nonlinear optics

Abstract

Although it is well known that light carries momentum and exerts a pressure on objects, a conservation of momentum principle is apparently rarely used in optics. In nonlinear optics light waves interact and may exchange both energy and momentum. We demonstrate that a conservation of momentum principle holds in these cases and in fact its use is widespread but generally unrecognized in the standard mathematical methods. In both the cases of linear basis waves interacting nonlinearly, e.g. coupled-wave theory and frequency mixing, and fully nonlinear waves, we demonstrate that a governing Hamiltonian is related to momentum. Action principles are used to discuss the generality of these results.     

The Effects of Temperature and Electric Field on a Quantum Rod Qubit

The Hamiltonian of a quantum rod (QR) with an ellipsoidal boundary is given after a coordinate transformation, which changes the ellipsoidal boundary into a spherical one. We then study the eigenenergies and the eigenfunctions of the ground and the first excited states of an electron strongly coupled to LO-phonon under an applied electric field by using variational method of Pekar type. This QR system may be used as a two-level quantum qubit. When the electron is in the superposition state of the ground and the first excited states, we obtained the time evolution of the electron probability density oscillating in the QR with a certain period. We then investigate the effects of the temperature and the electric field on the time evolution of the electron probability density and the oscillation period. It is found that the electron probability density and the oscillation period increase (decrease) with increasing temperature in lower (higher) temperature regime. The electron probability density decreases (increases) with increasing electric field when the temperature is lower (higher). The oscillation period decreases with the increase of the electric field along the ρ _x direction.